1. Field of the Invention
The present invention generally relates to surface treatments of substrate materials. More particularly, this invention relates to a process for treating the surface of a perovskite oxide substrate used for epitaxial growth of heterostructures, by which the quality of the epitaxial growth is promoted by changing the surface termination of the substrate.
2. Description of the Prior Art
Perovskite oxide single-crystal substrates are widely used for epitaxial growth of heterostructures, such as high-Tc superconductor films, Josephson tunnel junctions, superlattices, and oxide-channel field emission transistors (OxFET). As an example, strontium titanate (SrTiO3; STO) substrates have been used to grow high-Tc superconducting cuprate thin films such as YBa2Cu3O7-xcex4(YBCO). An atomically flat substrate surface is essential for accomplishing a perfect two-dimensional epitaxial growth from a perovskite substrate. In U.S. Pat. No. 5,855,668 to Kawasaki et al., single-crystal STO substrates with surface roughnesses of 5 Angstroms and less were obtained through a chemical etching process in buffered HF. Surface termination also plays an important role in film growth on perovskite substrates, including the ability to obtain perfect two-dimensional epitaxy of heterostructures. For example, it has been reported that pulsed laser deposition (PLD) of YBCO is strongly affected by the surface layer termination of STO (100) substrates. The surface termination of a perovskite substrate may additionally influence the electronic conductivity of thin films grown on perovskite substrates by such methods as PLD.
STO and other perovskite oxides used for epitaxial growth of heterostructures have the general formula AxAxe2x80x21-xBO3, where A is a rare earth metal (e.g., lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium), Axe2x80x2 is magnesium, calcium, strontium or barium, and B is a transition metal (e.g., titanium, vanadium, chromium, manganese, iron, cobalt, nickel and copper). Such perovskite oxides have two possible (100) surface termination layers, where the surface unit cell is either AO (A-site layer) or BO2 (B-site layer). In the case of STO, A-site terminations are SrO while B-site terminations are TiO2. In general, the methods that have been developed to achieve surfaces containing a single class of domain have been limited to producing B-site surface terminations, i.e., TiO2 for STO (100) substrates. For example, and as reported in T. Ohnishi et al., xe2x80x9cA-site Layer Terminated Perovskite Substrate: NdGaO3,xe2x80x9d Appl. Phys. Lett. 74, 14 (1999), it is well known that chemical etching of STO (such as Kawasaki et al.) results in TiO2 terminated surfaces. Unfortunately, undesirable CuOx precipitates have been found in cuprate heterostructures epitaxially grown on the TiO2 terminated surfaces of STO (100). A solution proposed in the prior art is to deposit a monolayer of SrO on a B-site terminated STO (100) substrate, which has been shown to result in a homogeneous layer-by-layer growth. For this purpose, a monolayer of SrO can be deposited, by ablating a strontium or strontium oxide target in an oxygen-containing atmosphere. However, significant drawbacks with this approach include the requirement for strict control of the deposition rate, and the possibility of undesired doping effects during epitaxial growth of certain oxides.
From the above, it can be seen that it would be desirable to provide a more facile method to produce A-site terminated perovskite substrates for epitaxial growth of heterostructures, such as high-Tc superconductor films, Josephson tunnel junctions, superlattices, and OxFET.
The present invention provides a method for changing the surface termination of a perovskite substrate surface, an example of which is the conversion of B-site terminations of a single-crystal STO substrate to A-site terminations. The method generally comprises etching the substrate surface by applying a reactive plasma thereto in the presence of low concentrations of halogens, during which B-site terminations are converted to A-site terminations. More particularly, the resulting substrate surface predominantly contains A-site surface terminations, i.e., SrO for STO (100) substrates. Because plasma etching leaves a disordered surface, the substrate is then preferably heated to a temperature sufficient to regenerate a long range order of the surface. A suitable heat treatment can effectively occur during the process of heating the substrate prior to depositing a film on the A-site terminated substrate surface. The proper surface termination contributes to a better long range order in a film epitaxially grown on the surface. In addition, disadvantages associated with B-site terminated perovskite substrate surfaces are avoided, including the propensity for undesirable CuOx precipitates found in cuprate heterostructures epitaxially grown on B-site terminated surfaces of STO (100).
A suitable etching treatment is a low power oxygen ashing in the presence of low levels of a halogen. Accordingly, the present invention provides a more facile method for producing A-site terminated substrates, as in comparison to the deposition of a monolayer of SrO on a B-site terminated STO (100) substrate, as proposed in the prior art. In addition, the present invention avoids unwanted doping effects in the epitaxial film caused by excess SrO, deficient stoichiometry, unwanted precipitates caused by insufficient strontium deposition, or faulty epitaxy caused by excess strontium deposition. Another advantage is that the solution provided by the present invention is part of the preparation of the substrate prior to deposition of an epitaxial film, and can therefore be more readily controlled. Consequently, heterostructures epitaxially grown on A-site terminated surfaces produced by this invention allow for the deposition of cuprate films with improved quality, which in turn renders such films more suitable for use as high-Tc superconductor films, Josephson tunnel junctions, superlattices and OxFET.
Other objects and advantages of this invention will be better appreciated from the following detailed description.